Electrical machines of a high-voltage type of design are usually formed with windings from preformed coils with flat wires (also known as profile wires) (cf. Sequenz H.: “Herstellung der Wicklungen elektrischer Maschinen”, Vienna, Springer-Verlag 1973, from page 107). An insulation that does not produce partial discharges for windings of such machines is described, for example, in DE-B 10 56 722, where the enameled profile wires were provided with a taping of micaceous insulating tapes and the windings were impregnated with impregnating agents by the vacuum impregnating process and cured. To allow insulated preformed coils of profile wires to be inserted into laminated cores, it is necessary however to arrange open slots in the laminated cores, which however has disadvantageous effects on the operating behaviour, or makes it necessary to close the slots laboriously with magnetic slot seals, to avoid these disadvantages. Moreover, the production of preformed windings with flat wires is labor-intensive and requires a high level of technological expenditure.
To avoid these disadvantages, it was proposed in DE-A-198 03 308 to arrange in half-open slots of the laminated core a slot lining of a micaceous sheet-like insulating material, to place in the slots a round-wire mush winding of a winding wire taped with film mica tape with one or more layers of overlap and to break down each coil respectively lying in two slots into at least two part-coils lying one over the other, arranged one behind the other and insulated from one another by intermediate layers. Half-open or half-closed slots are understood in this case as meaning slots with a slot slit of a width which is much smaller than the slot width, thereby dispensing with the effort of introducing magnetic slot seals.
It is known on the other hand that, with the use of pulsed voltage inverters for setting the speed, the problem of interturn or interphase shorts causing three-phase machines to fail, which had been regarded as solved, has occurred again in individual application situations. To avoid such failures, limit values of the winding loading were recommended (for example in the technical journal EMA, issue 3/96), although a reduction in the service life of the electrical machines has nevertheless to be expected or the installation of additional system components (for example filters) is necessary.
Therefore, EP-A1-0 822 640 proposed a low-voltage insulation that does not produce partial discharges for windings of electrical machines, in particular for frequency-converter-fed three-phase motors, which is characterized in that the winding comprises enameled wires known per se, in that these enameled wires are provided with a taping of micaceous insulating tapes, in that the slot is lined with sheet-like insulating materials of high heat resistance, in that the number of parallel wires and parallel branches is minimized and in that the windings are impregnated with impregnating agents by the power-UV process or the vacuum-impregnating process and cured. By minimizing parallel branches and/or conductors, it is intended to counteract the increase in the space requirement caused by the additionally introduced taping.
Today it is customary to operate low-voltage machines on frequency converters with high clock frequencies of up to 20,000 Hz. The problem lies with the occurrence of partial discharges between neighbouring conductors, caused by the steep voltage edges of the converter voltage. In the worst case, this has the effect of overvoltages, which may reach over twice the value of the voltage of the intermediate circuit. Since partial discharges may lead to the destruction of the enamel insulations of conventional winding wires, and thereby result in electrical breakdown, the use of enameled wires in converter-operated motors is problematical at supply voltages above 400 V.